The first millimetre interferometer in the Southern Hemisphere

The ATCA by night.

The Australia Telescope Compact Array (ATCA) has recently been upgraded by CSIRO to become the first millimetre interferometer in the Southern Hemisphere. ATCA, a telescope that traditionally operates at radio frequencies (ie. wavelengths of a few cm to a few tens of cm), will henceforth allow observations at wavelengths of 12 mm and 3 mm.

To date three of the six antennas have been equipped with the 3-mm receiver and the full array will be operational at 12 mm by May 2003. Once fully upgraded, the ATCA millimetre interferometer (ATCA-mm) will perform in the frequency ranges 16-26 GHz and 85-110 GHz (the 12 and 3 mm bands, respectively). ATCA-mm will be particularly suitable for studying processes in which molecular line emission at 12 mm (e.g. ammonia and water vapour masers) and 3 mm (e.g. methanol) are predominant.

The hot core G318.95-0.17, where HCO+, CH3OH (methanol), HCN and HC3N lines were detected and imaged. The HCO+ (contours) image reveal a bipolar structure centred at the position of the methanol masers (white cross). CH3OH emission (grey scale) is well confined in an unresolved hot molecular core. These results suggest that G318.95-0.17 harbours a very young massive stellar object, and perhaps a massive protostar.

The star-forming group in the School has actively participated in the commissioning of ATCA-mm, and has also submitted numerous observational proposals for the new facility this year. Our goals essentially focus on the study of the formation of the most massive stars in the Galaxy through molecular line surveys toward methanol maser sources. Methanol masers are recognised as a likely exclusive signpost of regions of massive star formation. More interestingly, radio observations toward a large source sample have shown that methanol masers are isolated from traditional signposts of massive star formation such as ultra-compact HII (UCHII) regions. A recent Mopra molecular line survey toward the so-called isolated methanol maser sources revealed that they could signpost hot molecular cores that are weakly or non-ionised; i.e. a phase of massive star formation earlier than the UC HII phase. Observations with the newly upgraded ATCA at 3 and 12 mm have been undertaken toward the best hot core candidates from our Mopra survey (see figure).

In total, our group have secured about 150 hours with ATCA-mm and become the leading group in the emerging field of millimetre astronomy in Australia.

Vincent Minier




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